1. Field of the Invention
The invention relates to optical materials testing, and particularly for optical materials of components such as prisms, windows or etalons of a resonator arrangement of a gas discharge laser system such as an excimer or molecular fluorine laser.
2. Discussion of the Related Art
Integrated circuit device technology has entered the sub 0.25 micron regime, thus necessitating very fine photolithographic techniques. The reduction in size of a structure produced on a silicon wafer is limited by the ability to optically resolve the structure. This resolution ability depends directly upon the photolithographical source radiation and optics used.
Excimer lasers emitting pulsed UV-radiation are becoming increasingly important instruments in specialized material processing. The term xe2x80x9cexcimerxe2x80x9d was first utilized as an abbreviation for xe2x80x9cexcited dimerxe2x80x9d, meaning two or more identical atoms comprising a molecule which only exists in an excited state, such as Ar2 and Xe2. Today, the term xe2x80x9cexcimerxe2x80x9d has a broader meaning in the laser world and encompasses such rare gas halides as XeCl (308 nm), KrF (248 nm), ArF (193 nm), KrCl (222 nm), and XeF (351 nm). Additionally, F2 (157 nm) may be used as an active media within excimer laser discharge chambers. The molecular fluorine (F2) laser is in fact typically referred to as an excimer laser, and thus when the term excimer laser is used in this application, it is intended that the molecular fluorine laser be included within the meaning of that term.
As is apparent, many excimer lasers radiate at ultraviolet wavelengths making them desirable for use as lithography tools. The KrF-excimer laser emitting around 248 nm and the ArF-excimer laser emitting around 193 nm are rapidly becoming the light sources of choice for photolithographic processing of integrated circuit devices (IC""s). The F2-laser is also being developed for such usage and emits light around 157 nm.
To produce smaller feature sizes on IC chips, stepper and scanner machines are using expensive large scale submicron projection objectives for imaging a reticle onto a wafer surface with high diffracting-limited precision. The objectives operate at deep ultraviolet (DUV) wavelengths, such as the emission wavelengths of excimer lasers. For example, the KrF-excimer laser emitting around 248 nm is currently being used as a DUV radiation source. To reach greater resolution limits, the large field objective lenses are designed and optimized in view of various possible and discovered imaging errors. The design optimization of the objectives is, however, inadequate to meet the precision demands of sub-quarter micron lithographic technology.
One way to improve the resolvability of structures on IC chips is to use more nearly monochromatic source radiation, i.e., radiation having a reduced bandwidth, xcex94xcex. Other strategies include using shorter absolute wavelength, xcex, radiation such as that emitted around 193 nm and 157 nm by ArF- and F2-lasers, respectively, and increasing the numerical aperture (NA) of the projection optics.
The smallest structure resolvable on an IC chip depends on the xe2x80x9ccritical dimensionxe2x80x9d (CD) of the photolithography equipment being used:
CD=K1xc2x7xcex/NAxe2x80x83xe2x80x83(1), 
where NA is the numerical aperture and is a measure of the acceptance angle of the projection optics, xcex is the wavelength of the source radiation, and K1 is a constant around approximately 0.6-0.8.
Simply increasing the numerical aperture NA to reduce the critical dimension CD according to (1), however, simultaneously reduces the depth of focus DOF of the projection lens by the second power of NA:
DOF=K2xc2x7xcex/(NA)2xe2x80x83xe2x80x83(2), 
where K2 is a constant around approximately 0.8-1.0. Reducing the DOF complicates wafer adjustment and adds further demand for improved chromatic correction of the projection lenses. Additionally, increasing the numerical aperture NA to reduce the critical dimension CD for achieving smaller structures requires a decrease in the bandwidth xcex94xcex of laser emission according to:
xcex94xcex=K3xc2x7xcex/(NA)2xe2x80x83xe2x80x83(3), 
where K3 is a constant dependent on parameters associated with the projection optics. Each of the above, i.e., (1), (2) and (3), assumes that such other laser parameters as repetition rate, stability, and output power remain constant.
To produce smaller features on silicon substrates, the projection optics may be modified to increase the numerical aperture of the system or the bandwidth of the exposure radiation may be reduced, as discussed. For the reasons provided above, it is most advantageous to use a shorter wavelength exposure radiation source to facilitate smaller feature size production, in combination with narrowing the bandwidth and using projection optics of appropriate numerical aperture.
Another significant feature of a desired narrow band laser to be used advantageously for resolving small features on chips is that the laser system also exhibits high absolute wavelength stability. For example, a laser output beam wavelength stability around or below 0.1 pm is desired.
It is also desired to operate the laser at a high repetition rate. For example, it is desired to have a laser operating above 1 kHz, and particularly around 2 kHz, or more. At higher operating power, though, the amount of induced absorption at transmissive optical component in the laser resonator correspondingly increases.
Increased absorption leads to undesirable reduced wavelength stability since the temperatures of the optical components within the resonator influence the range of wavelengths that fall within the acceptance angle of the resonator. For example, the refractive index of a prism depends on its temperature and influences its wavelength dependent refractive properties. Absorption in etalon plates can affect the effective etalon finesse and can increase the laser radiation bandwidth. A window, plate or other transmissive or reflective component in the resonator can undergo surface distortions as its temperature fluctuates that can result in a degradation of many significant beam parameters. The effect is pronounced for laser systems operating in burst mode, since the components absorb significantly during a burst and heat up, and then cool down during pauses between bursts.
It is desired, then, to minimize the amount of absorption of optical components in the laser resonator. One way is to test the components in advance of placing them in the laser resonator to ensure their absorption constants are sufficiently low. U.S. Pat. No. 5,894,352 to Morton, which is hereby incorporated by reference into the present application, discloses an apparatus and method for performing such testing of prisms. Morton""s technique arranges several prisms in series each having a temperature sensor in its vicinity for measuring its temperature. Then, a beam is passed through the prisms and the temperatures measured with the sensors. Those prisms that perform poorly such as by exhibiting an undesirably high rate of increase in temperature due to absorption are advantageously sorted out from satisfactory prisms before they are used in a laser system.
It is recognized in the present invention that a drawback of conventional techniques is that significant time and cost of manufacturing the prisms or other optical components that are ultimately thrown out as a result of the test is wasted. It is also recognized in the present invention that the material block from which prisms, etalon plates and other optics are formed generally exhibits either a high or low absorption performance before the optics are even manufactured from the material block. It is desired to preserve the time and cost that are wasted in conventional testing techniques that manufacture optical components, such as may be subject to precise specifications, only to determine later on that the material exhibits unsatisfactory absorption performance. It is also desired to have a narrow band laser system for photolithographic production of fine structures that operates at a high repetition rate and exhibits satisfactory wavelength stability partly because the optical components of the system do exhibit satisfactory absorption performance.
It is an object of the invention to provide a high repetition rate (e.g., 1-2 kHz or more), narrow band excimer or molecular fluorine laser system that exhibits high wavelength stability (e.g., less than 0.2 pm, and preferably below 0.1 pm).
It is also an object of the invention to provide the laser system with internal optics that exhibit satisfactory absorption performance in accord with the first object by performing absorption testing prior to insertion of the optics into the laser resonator.
It is a further object of the invention to minimize the time and cost spent on optical components comprising materials that exhibit unsatisfactory absorption performance.
In accord with the above objects, a method is provided for testing a material block prior to forming the material block into one or more optical components for use with a sub-micron lithographic, high power, narrow bandwidth laser system having high wavelength stability (e.g., within 0.1 pm for a sub-picometer bandwidth output beam). The method includes the step of selecting a block of material having appropriate characteristic optical properties such as minimal absorption coefficient or excellent optical homogeneity, at the output emission wavelength(s) of the DUV (e.g., 248 or 193 nm), or VUV (e.g., 157 nm) source laser system being used. For this purpose a material such as fused silica, MgF2 and/or CaF2 (preferred) may be selected for a DUV lithography system, and CaF2 may be selected for use with a VUV system.
The next step of the method of the present invention is to test the material block for absorption performance. Then, if the block exhibits an absorption performance above a predetermined value, then one of more optical components such as one or more prisms, etalons, and/or windows, etc. are formed from the block. Finally, the optical components formed from the block are inserted into the laser resonator to participate in producing a high power, narrow bandwidth laser beam which may be used in sub-micron photolithographic applications. Advantageously, optical components are only formed from material blocks tested as exhibiting sufficient absorption performance, which saves time and cost.